In networks, medium access control enables multiple devices to share the medium for communication. The same applies within a wireless communication medium in which multiple devices form a wireless network. Numerous medium access control strategies for wireless networks exist. Generally, most of those medium access control strategies may be classified into one of the 3 broad categories, namely, Frequency Division Multiple Access (FDMA), Time Division Multiple Access (TDMA) and Code Division Multiple Access (CDMA).
FDMA is a strategies wherein the frequency spectrum is partitioned into frequency channels, and the frequency channels are assigned to users. With FDMA, only one user at any given time is assigned to a particular frequency channel. FIG. 1 illustrates the manner in which FDMA partitions the frequency spectrum into N frequency channels, such as frequencies channels 101, 102 and 103.
In contrast to FDMA, the TDMA strategies partitions the medium access time into time slots. Spectrum capacity is improved by allowing each user to access the entire frequency spectrum for the short period of time. Other users share the same frequencies within the frequency spectrum, but at different time slots. FIG. 2 illustrates the manner in which TDMA partitions the medium access time into N time slots, such as time slots 201, 202 and 203.
CDMA increases spectrum capacity by allowing all users to occupy all frequency channels at the same time. Each user's transmission is assigned a unique code to differentiate that transmission from the other users' transmissions. FIG. 3 illustrates the manner in which CDMA allows users to occupy the entire frequency spectrum all the time, but how different transmissions are allocated different codes, such as codes 301, 302 and 303.
In practical wireless networks some combination of FDMA, TDMA and CDMA is usually used. FDMA and CDMA is usually employed in the Physical (PHY) Layer of the devices in the wireless network, whereas TDMA is usually used in the Medium Access Control (MAC) layer, which is above the PHY layer.
Various TDMA MAC protocols exist. One of the earliest TDMA MAC protocols developed is the Aloha protocol. In the Aloha protocol, any source device wanting to transmit data simply transmits and waits for an acknowledgement from the destination device. If the data was not successfully received due to a collision with another transmission, the source device simply retransmits the data later.
An enhancement to the Aloha protocol is known as the Slotted Aloha protocol. In this protocol the medium access time is divided into fixed interval slots. When a source device wants to transmit, that device transmits at the earliest slot interval and waits for the acknowledgement as in the case of the (normal) Aloha protocol. Again, if the data was not successfully transmitted due to a collision with another transmission, the source device again retransmits the data later. However, in requiring devices to transmit only in slot intervals, transmission collisions are restricted to complete packet collisions only, thereby eliminating partial packet collisions which often occur when the (normal) Aloha protocol is used.
Another MAC layer TDMA protocol is that of the Institute of Electrical and Electronic Engineers (IEEE) Wireless Local Area Network (WLAN) standards 802.11. In the IEEE 802.11 MAC standard, the medium access time is partitioned into a regular time interval, called a Target Beacon Transmission Time (TBTT). FIG. 4 shows the medium access time, and TBTTs 401, 402, 403 and 404. At or following each TBTT a special packet, called a beacon frame, is broadcasted. FIG. 4 also shows the beacon frames, such as beacon frames 441 and 412 transmitted at or following the TBTTs 401 and 402 respectively. It is noted that according the IEEE 802.11 MAC standard uses contention for broadcasting the beacons frames 411 and 412, causing the beacon frames to be delayed if there is existing transmission during the TBTT. The beacon frames are used to synchronize all devices in the network, as well as to provide other important control information of the network.
Depending on the mode in which the IEEE 802.11 MAC standard is implemented, the device that broadcasts the beacon frames may differ. In the Infrastructure mode (centralized-based mode) of the IEEE 802.11 standard, only the Access Point (AP) device broadcasts a beacon frame during each TBTT. In the Independent Basic Service Set (IBSS) mode (also known as ‘Ad-hoc mode’), every device in the network will attempt to broadcast a beacon frame during each TBTT. However, through contention, only a single device will be able to successfully send out a beacon frame at each TBTT. During other times other that the TBTTs, the devices share the medium access time using either a ‘Distributed Coordination Function’ (DCF) or a ‘Point Coordination Function’ (PCF). The DCF uses a commonly known ‘Carrier Sense Multiple Access with Collision Avoidance’ (CSMA/CA) technique. In order to provide for prioritized medium access, the IEEE 802.11 standard provides different ‘Interframe Spaces’ (IFSs) of varying duration to be used for delay before performing the backoff required to contend for medium access. The 4 different IFSs are ‘Short Interframe Space’ (SIFS), ‘PCF Interfame Space’ (PIFS), ‘DCF Interframe Space’ (DIFS) and ‘Extended Interframe Space’ (EIFS). FIG. 5 illustrates medium access using the DCF according to the IEEE 802.11 standard, and the relationships between the SIFS, the PIFS and the DIFS.
FIG. 6 illustrates medium access using the PCF according to the IEEE 802.11 standard. The PCF is a contention-free transfer protocol based on a polling scheme controlled by a Point Coordinator (PC) operating at the AP. The PC gains control of the medium at the beginning of the contention-free period (CFP) and attempts to maintain control for the entire CFP by waiting a shorter time between transmissions than other devices using the DCF access procedure.
Besides the IEEE 802.11 WLAN standards, IEEE has also defined standards for a Wireless Personal Area Network (WPAN). One such WPAN standard is the IEEE 802.15.3 High Rate WPAN standard. In the IEEE 802.15.3 WPAN standard TDMA is also employed in the MAC layer. The medium access time is partitioned into periodic superframes. The IEEE 802.15.3 WPAN standard defines a centralized controlled topology as its network topology. Devices generally may be classified as being a normal operating device (DEV), or a device may assume the role of a Piconet Coordinator (PNC). The PNC broadcasts a beacon frame once every superframe. Any DEV that receives the beacon frame may choose to join the PNC's network, termed a piconet, hence forming a centralized controlled network that centres about the PNC. FIG. 7 illustrates a PNC 701 and DEVs 702 to 705 in wireless receiving range of the PNC. Hence, DEVs 702 to 705 are able to receive the beacon frame broadcasted by the PNC 701, while all devices 701 to 705 may exchange data within the piconet.
As is illustrated in FIG. 8, the superframe defined by the IEEE 802.15.3 standard is further partitioned into a beacon slot, a Contention Access Period (CAP) and a Channel Time Allocation Period (CTAP). The beacon slot is used by the PNC to broadcast a beacon frame without any contention. The CAP is used by the PNC and the DEVs for transmission of command/response or for contention-based traffic. The medium access time within the CTAP is divided into multiple slots reserved by the PNC for contention-free communication from DEVs.
The IEEE has further defined a standard for a WPAN which is targeted towards low rate devices. That standard is the IEEE 802.15.4 Low Rate WPAN standard. In that standard, 2 types of devices are defined, namely a Full Function Device (FFD) and a Reduced Function Device (RFD). Depending on the application requirements, the standard may operate in either of 2 topologies, namely a star topology or a peer-to-peer topology. FIG. 9 illustrates the star and peer-to-peer topologies of the IEEE 802.15.4 Low Rate WPAN standard, the position of the Personal Area Network (PAN) coordinator within those topologies, and communication flow between devices.
The IEEE 802.15.4 Low Rate WPAN standard uses TDMA in the MAC layer similar to the IEEE 802.15.3 High Rate WPAN standard. In the IEEE 802.15.4 standard devices may share the medium access time using the simple CSMA/CA technique. Optionally, superframe structures may be used. The format of the superframe is defined by the coordinator. As is illustrated in FIG. 10, the superframe is divided into 16 equally sized slots. Superframes are bounded by network beacons, which are sent by the coordinator.
For low-latency applications or applications requiring specific data bandwidth, the coordinator may dedicate portions of the active superframe to that the devices executing those applications. Such dedicated portions are termed Guaranteed Time Slots (GTSs). The GTSs form the CFP. As is illustrated in FIG. 11, the CFP always appears at the end of the active superframe starting at the slot immediately following the CAP.
Yet another High Rate WPAN protocol is that defined by the Multi-Band OFDM Alliance (MBOA) group. In order for every device in the WPAN to be able to form its own network, each device is required to broadcast a beacon frame in a distributed fashion. The MBOA MAC v0.93 standard defines a superframe of a device to have duration of 65536 μs. FIG. 12 illustrates a superframe according to the MBOA MAC v0.93 standard. The superframe is composed of 256 Media Access Slots (MAS) where each MAS has a duration of 256 μs. The first part of the superframe is reserved for beacon frame broadcasting. The number of MASs that is actually used for beacon frame broadcasting is defined as the Beacon Period (BP). The BP is sub-divided into Beacon Slots (BSs). The rest of the MASs in the superframe are used for data transfer, employing either a prioritized contention-based method, called Prioritized Channel Access (PCA), or a data reservation method, called Distributed Reservation Protocol (DRP). The BP is dynamic in length and consists of a dynamic number of BSs. The BP expands when new devices join the Beacon Group (BG) and contracts when devices leave the BG. The BG is defined as a group of devices which synchronizes their beacon frame transmissions within the same group of MASs and which identify these MASs as their BP. When two or more BGs come into range of each other, devices are required to coalesce to a single BP, combining into a single BG. The BP of one of the BG will be expanded to accommodate beacon frames of other joining devices from other BGs.
In practical usage scenarios, many wireless medium access control issues are present. One such an issue is the issue of mobility. Depending on applications and the nature of the devices within a wireless network, the network may either have a static topology or a dynamic topology whereby devices enter and leave the network frequently. For dynamic topologies, due to the mobility of devices in the network, a certain degree of ad-hoc connectivity has to be supported by the medium access control. In addition, due to devices' mobility, transmissions from a source device, with such transmissions being either broadcasts of beacon frames or transmission of data frames, may have a high probability of collision with the transmission of another device within the network. For MAC employing beacon frames for synchronization as well as to broadcast control information, beacon frames broadcast by 2 or more devices may collide due to mobility.
An example topology where this may occur is illustrated in FIG. 13 where devices 1302 and 1304 broadcast beacon frames. Devices 1301, 1303, 1305 and 1306 listen for such broadcasted beacon frames. If devices 1302 and 1304 chose the same time slot to broadcast their respective beacon frames, and those devices 1302 and 1304 are in positions such that a device, such as device 1303, is in broadcast range of both devices 1302 and 1304, then the beacon frames broadcast from the devices 1302 and 1304 will collide. Both devices 1302 and 1304 will be unable to detect this collision as reception during transmission is typically not provided for. In order for a device to receive at the same time as that device transmits required extra complexity to be employed, which includes the implementation of multiple antennas. It is assumed that beacon frames are sent without contention, as contention will result in delays caused by contention backoff, causing the timeliness of the beacon frames not to be guaranteed. The fact that device 1303 is unable to receive the beacon frames of both devices 1302 and 1304 is used by certain MAC designs to provide feedback to devices 1302 and 1304 to inform those devices that their beacon frames are not properly received, thus indicating that there may be a beacon frame collision. Devices 1302 and 1304 may then take action to change to other time slots to broadcast their respective beacon frames.
However, consider the case illustrated in the topology shown in FIG. 14 where the devices broadcasting beacon frames are devices 1402 and 1403. In this case there is no other device in broadcasting range of both of the beacon frame broadcasting devices 1402 and 1403. The beacon frame broadcasting devices 1402 and 1403 are unable to detect any beacon frame collision because they use the same beaconing slot to broadcast their beacon frames. Hence, devices 1402 and 1403 are unable to discover each other, and consequently unable to communicate with each other although they are in wireless range.
Another common issue for wireless medium access control is the problem of Simultaneous Operating Piconet (SOP). The SOP problem is very often encountered by MAC design based on centralized control. FIG. 15 shows a wireless network topology including 3 device 1503, 1508 and 1509 broadcasting their beacon frames. The wireless ranges of those devices 1503, 1508 and 1509 are indicated by the boundaries 1512, 1513 and 1514 respectively. In a centralised model where devices 1503, 1508 and 1509 act as central coordinators, devices 1504 and 1506 may be unable to communicate with each other as those devices 1504 and 1506 are connected to different central coordinators 1503 and 1508 respectively, although they may be in wireless range of each other.
Another SOP problem exists for beaconing frame broadcasting devices 1508 and 1509. When 2 beacon frame broadcasting devices exist in the same wireless space, like in the case of devices 1508 and 1509, one possibility is that one of the devices 1508 or 1509 will have to join the other network. Alternatively, some additional protocol is required to ensure such devices can coexist together.
Quality of Service (QoS) is also an issue for wireless medium access control. In case of low latency applications or applications requiring specific data bandwidth some means to provide for guaranteed time access is required. To provide for QoS, contention-based medium access is not appropriate as medium access is not guaranteed. This is because contention based medium access is subjected to time delays for medium sensing, random backoff and collision.
The Aloha protocol discusses above has the problem of being unable to provide QoS as the collision of data packets is very common.
As for the IEEE 802.11 MAC standard, the infrastructure mode does not support mobility of the network, as that standard requires a static AP. If the AP is moved out of the wireless network, or switch off, the entire network collapses. As for the IBSS mode, although mobility is supported, beacon frames are transmitted through contention, and for reasons described above, this solution is subject to time delays. Another downside is that only a single beacon frame can be transmitted once in each superframe. This means that in a network of many devices, it may take a long time to discover a particular device, subjected to whether that device can successfully content for the medium to broadcast its beacon frame.
For IEEE 802.15.3 High Rate WPAN, although support for QoS is present via CTAP reservation, the protocol is also based on centralized control. Similar to the infrastructure mode of the IEEE 802.11 standard, if the PNC is moved out of the wireless network, or suddenly powered off, the network will temporarily cease functioning. However in the IEEE 802.15.3 standard, another device can resume the role of PNC, thus providing a mean for the network to continue functioning. The main problem with the IEEE 802.15.3 standard is that of SOP discussed with reference to FIG. 15.
As for the IEEE 802.15.4 standard, the problem of beacon frame collision discussed with reference to FIGS. 13 and 14 is present in wireless networks according to that standard. In addition, that standard makes no provision for multiple devices to broadcast beacon frames in the same wireless space.
The problem of beacon frame collision also occurs in wireless networks according to the MBOA MAC v0.93 standard. In fact, the problem may be worse in the case of the MBOA MAC v0.93 standard as every device in the network is mandated to broadcast beacon frames. This is undesirable, especially in the case of battery-powered slave devices as this solution causes such devices to consume additional power, or in the case of devices choosing to stay in a passive mode. In addition, the additional dynamic BP contraction, expansion and merging procedure required poses additional complexity as well as power consumption.
As can be seen from the problems associated in various MAC protocols, a need still exists for a relatively low complexity medium access protocol that provides for mobility, SOP and QoS.